<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="../css/reset.css">
    <style>
        /* html {
            height: 100%;
        }

        body {
            height: 100%;
        } */

        /* html,
        body {
            height: 100%;
        }

        .a {
            width: 100%;
            height: 100%;
            background-color: #f00;
        } */

        .a {
            position: fixed;
            left: 0;
            right: 0;
            top: 0;
            bottom: 0;
            background-color: #ff0;
        }
    </style>
</head>

<body>
    <div class="a"></div>

</body>

</html>